1. Field of the Invention:
The present invention relates to a signal processing device, and particularly to an audio signal processing circuit which inputs subband samples of audio signals that have been divided and quantized in advance for each frequency band, processes these signals and outputs the results as audio samples.
2. Description of the Related Art:
ISO/IEC 11172-3 contains provisions for the international standards for an expansion circuit for compressed audio signals in such an audio signal processing circuit. An example of the construction of this type of audio signal processing circuit of the prior art is shown in FIG. 1. The circuit shown in FIG. 1 carries out a discrete cosine transform (DCT) of the subband signal, combines bands, and generates an audio signal.
This audio signal processing circuit is composed of requantizing circuit 12; DCT memory 14 for storing DCT samples for discrete cosine transform; DCT circuit 16 for carrying out discrete cosign transforms; band synthesis memory 18; address generating circuit 44; band synthesis circuit 20; and control circuit 43 for controlling the operation of these circuits.
Requantizing circuit 12 requantizes a quantity N (where N is an integer product of 4) of input subband samples 1, which have been quantized to differing word lengths for each frequency, for every group of 32 samples, and stores the requantized 32 DCT samples 13 in DCT memory 14.
DCT circuit 16 applies a discrete cosine transform represented by the following formula (1) to DCT samples 15 stored in DCT memory 14, and stores the 32 band synthesis samples 17 comprising from syn[0] to syn[31] resulting from the transform into band synthesis memory 18. ##EQU1##
Address generating circuit 44 generates addresses in band synthesis memory 18 in which band synthesis samples 17 calculated by DCT circuit 16 are stored. Band synthesis memory 18 has a storage area for 512 samples made up of 16 blocks of 32 samples each.
The block for storage of each discrete cosine transform is successively shifted such that, when the first 32 samples resulting from discrete cosine transform are stored in the first block, the next 32 samples resulting from discrete cosine transform are stored in the second block, and when samples have been stored in all 16 blocks, the next results of discrete cosine transform are again stored in the first block.
Band synthesis circuit 20 extracts one sample from each block of data stored in band synthesis memory 18 for each fixed time period determined as the sampling period for a total of 16 samples, carries out a product/sum operation, and outputs the result of this operation as output audio sample 11 for one sample.
Regarding the overall operation of this audio signal processing circuit, when 32 quantized input subband samples 1 are inputted to requantizing circuit 12, requantizing circuit 12 first requantizes this input and then stores the resulting 32 DCT samples 13 into DCT memory 14.
DCT circuit 16 carries out the DCT operation on DCT samples 13 stored in DCT memory 14, and stores the resulting 32 band synthesis samples 17 (from syn[0] to syn[31]) in one block within band synthesis memory 18.
Thirty-two output audio samples 11 (from pcm[0] to pcm[31]) are calculated after the completion of the nth DCT processing and the storage of band synthesis samples 17 in band synthesis memory 18. For every calculation of one output audio sample 11, a total of 16 band synthesis samples 19 are extracted, one sample being extracted from each block of band synthesis memory 18 for use in this calculation.
The ith output audio sample pcm[i] of the 32 output audio samples 11 is calculated according by the following formula (2): ##EQU2##
In a case in which band synthesis circuit 20 calculates output audio sample 11 immediately after the results outputted from DCT circuit 16 are stored in band synthesis memory 18, for example, in a case in which pcm[i] is calculated in order for i=0 to i=31 immediately after the 16th DCT results are stored in block 16, the band synthesis samples necessary for the calculation are as shown by the half-tone portions in FIG. 2(a).
Next, the band synthesis samples necessary for calculation of pcm[i] of output audio sample 11 immediately after storing the 17th DCT results in band synthesis memory 18 are as shown by the half-tone portions in FIG. 2(b).
At the time of the state shown in FIG. 2(a), the data of from syn[17] to syn[31] of block 1 will not be used again. Based on formula (2), the memory is vacated successively beginning from syn[0] of block 1 after calculating pcm[16]. In other words, syn[0] is no longer necessary after calculating pcm[16], and syn[1] is no longer necessary after calculating pcm[17]. Processing can be continued by again storing band synthesis samples 19 in these areas that successively become unnecessary.
FIG. 3 is a timing chart showing the relation between the requantizing process, the DCT process, and the synthesizing process, and shows the calculation of 32 output audio samples 11 up to the beginning of the calculation of the next 32 output audio samples.
Band synthesis samples are stored in blocks 1 to 16 of band synthesis memory 18. With the start of the calculation of pcm[0] (35 in FIG. 3), requantizing circuit 12 first requantizes input subband samples 1 and stores the results in DCT memory 14 (36 in FIG. 3).
At this time, the portions in which from syn[17] to syn[31] were stored in block 1 of band synthesis memory 18 are vacated, and after calculating pcm[16], become vacant in order from syn[0] to syn[1] to syn[2], and so on. DCT circuit 16 therefore proceeds calculating and storing syn[i] in the order of this vacating of the memory (37 in FIG. 3).
This configuration of the prior art requires memory for 32 samples in DCT memory 14 (a total of 768 bits if each sample comprises 24 bits) and for 512 samples in band synthesis memory 18 (a total of 12,288 bits).